A CBS News investigation has revealed a startling development: foreign workers making $10 an hour are replacing American workers who would otherwise make $45-$50 an hour building auto plants in the US.

"Foreign automakers have received billions of dollars in subsidies and tax incentives to build factories in the U.S. and create American jobs.

A new CBS News primetime series, 'CBSN: On Assignment,' uncovers the hidden foreign workforce being used to build sections of U.S. auto plants.

This CBSN investigation took us thousands of miles, from South Carolina to Slovenia, in eastern Europe, in search of answers.

For three years, Gerald Greiner managed safety on American construction projects for a German contractor called Eisenmann. His first job was at Mercedes in Vance, Alabama in 2013.

'There was Polish and Slovenian and Croatian people there,' he told correspondent Vladimir Duthiers. 'It was hard for me to believe because I just didn't understand why they would be here.'

What did they do? 'Anywhere from steel erection to pipe fitting to pouring concrete to installing equipment, just about everything.'

The cars would be built by American workers -- but the building of the auto plants was being done by foreign workers. 'Exactly,' said Greiner. 'They come in at groundbreaking, they're done at start of production.'"